Abstract

Music boxes, bicycles, and clocks all have one thing in common: GEARS! You might say that gears make the world turn, since they are in so many mechanical instruments. How do they work and how do you know which gears to use? Find out in this simple experiment.

Objective

In this experiment you will count the number of teeth on gears and figure out how to calculate gear ratios by putting the gears together.

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Introduction

What exactly do gears do? They crank, mesh, pump, push, pull, tug, and grind. All of which turn out to be very useful for doing work. Many mechanical contraptions and gizmos use them, but how do they do work? The LEGO Education Connection explains:

Moving gears (Public Domain).

"A gear is a wheel with teeth on its outer edge. Gears rotate on a central axis and work with other gears to transmit turning force. The teeth of one gear mesh with—or engage—the teeth of another gear.

"The rotating force produced by an engine, windmill, or other device often needs to be transferred or changed in order to do something useful. For example, as you pedal a bicycle, you cause the sprocket to rotate. But in order to make the bike move, this rotating energy must be transmitted to the rear wheel.

"Gears are used to transmit turning force. They can also change the amount of force, speed, and direction of rotation." (LEGO, 2007)

Gears are generally used for one of four different reasons (Brain, 2000):

To reverse the direction of rotation

To increase or decrease the speed of rotation

To move rotational motion to a different axis

To keep the rotation of two axes synchronized

All of this, of course, depends on how the gears fit together. The most important thing to consider when putting two gears together is the gear ratio. This is a way of expressing the size of one gear relative to another. For example, LEGO makes gears in the following sizes: 40-tooth, 24-tooth, 16-tooth, and 8-tooth. An example of how to figure gear ratios follows (Genalo, 2000):

Line the first set of gears in front of you. Make a ratio of the number of teeth in the first gear against the number of teeth in the second gear. For example, if you have a 24-tooth gear and an 8-tooth gear: 24:8 = 3:1 The ratio would be 24:8. Find the greatest common denominator of this ratio and use it to simplify the ratio. The simplified ratio would be 3:1.

In this experiment you will use a set of toy gears to figure out an alternate way to determine the gear ratio. You will also try to figure out how the gear ratio will affect the revolutions per minute of two gears that are meshed together. After figuring this out your inventions will only need a dab of imagination!

Terms and Concepts

To do this type of experiment you should know what the following terms mean. Have an adult help you search the Internet, or take you to your local library to find out more!

This site describes gears in more general terms and has a nice animation of two gears turning: Brain, Marshall, 2000. "How Gear Ratios Work ," HowStuffWorks.com [accessed February 2, 2007] http://www.howstuffworks.com/gears.htm

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Materials and Equipment

a set of toy gears that fit together and come in a variety of sizes; Here are a few good ones that are available at national retailers like Amazon, Toys-R-Us, or Target:

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Experimental Procedure

For this experiment you will need a data table to write all of your numbers:

Smallest <============> Largest

Gear:

A

B

C

D

E

F

Teeth of the Gear

Teeth of Gear A

Ratio of Teeth

Turns of the Gear

Turns of Gear A

Ratio of Turns

Sort through your gear set to find a series of gears in different sizes that all fit together.

Put the gears out on the table in order from smallest to largest.

Place a small sticker or piece of colored tape next to ONE tooth of each gear, choosing a different color for each gear to help you keep track. Label each sticker with a letter (A, B, C, D, etc.) that you will use to identify that particular gear in your data table.

Using a permanent black marker, color that SAME TOOTH black so that it stands out and is easy to see. Don't worry about the marker being permanent, you can wipe it off later with some rubbing alcohol.

Count the number of teeth on each gear and write the numbers in your data table.

To calculate a ratio, divide the number of teeth for each gear (found in the 1st data row of your table) by the number of teeth on Gear A (found in the second data row of your table). Write your answer in your data table (in the 3rd data row of this table).

Check your math. Your ratio for Gear A should be 1:1, or 1.0 if you are using a calculator. If it is not be sure to check your math to be sure you are doing it right.

Starting with the smallest gear (A), attach it to the next larger gear in size (B) using the connectors in your toy gear kit.

Rotate until the two teeth marked with black permanent marker are directly next to each other. This will be your starting point.

Now rotate the smallest gear slowly in one direction until the two marked teeth touch together at exactly the same point as before. Count the number of rotations of both gears as you go from start to finish. There may be a lot of turns to count! You can either find a volunteer to count the other gear for you (best way) OR you can complete this step twice, counting one of the gears each time from start to finish (most tedious way). Write the number of turns that each gear makes into your data table.

Repeat this step with each of the larger gears (D, E, etc.) connected to the smallest gear (A) counting the number of revolutions for both gears each time.

Again, calculate a ratio by dividing the number of turns of each gear (found in the 4th data row of your table) by the number of turns of Gear A (found in the 5th data row of your table). Write the answers in your data table (for example, the last row of this sample data table).

Compare your two ratios. Are they similar or different? What does this tell you about how revolutions per minute are related to gear ratios?

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Variations

In this experiment you used the number of turns to calculate a gear ratio, by simplifying a fraction. Can you use the number of teeth in the two gears to get another fraction that will simplify the same way? Will this also calculate the gear ratio?

In this experiment you tested the smallest gear against each of the larger gears. A more advanced project is to test all possible combinations of gears. Here is one way to make a table to help you figure out what all of the possible combinations are:

A

B

C

D

E

A

AA

AB

AC

AD

AE

B

-

BB

BC

BD

BE

C

-

-

CC

CD

CE

D

-

-

-

DD

DE

E

-

-

-

-

EE

Attention gear-heads! If you like gears and bikes, then this might be of interest to you. You can count the number of teeth on each gear of your bike and see if it relates to the rpm of your wheels. Just prop up your bike and crank the pedals with your hand at a constant speed. Then have someone help you count the number of turns the wheels make in a minute to get the rpm's. Check out a short description in the Additional Projects section on the Applied Mechanics interest area page.

More advanced students can order specialized gears and calculate more complex gear ratios. You can also calculate how gearing up or gearing down will change the revolutions per minute. How is this useful for applied mechanics?

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Ask an Expert

The Ask an Expert Forum is intended to be a place where students can go to find answers to science questions that they have been unable to find using other resources. If you have specific questions about your science fair project or science fair, our team of volunteer scientists can help. Our Experts won't do the work for you, but they will make suggestions, offer guidance, and help you troubleshoot.

Related Links

If you like this project, you might enjoy exploring these related careers:

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Mechanical Engineering Technician

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